An electric power steering apparatus which provides a steering apparatus of an automobile or a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque (the steering assist torque), such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected value of the motor current becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of such an electric power steering apparatus will be described with reference to FIG. 1. A column shaft 2 connected to a steering wheel 1 is connected to tie rods 6 of steered wheels through reduction gears 3, universal joints 4a and 4b, and a rack and pinion mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a motor current Im of an assist command based on a steering torque T detected by the torque sensor 10 and a velocity V detected by a velocity sensor 12, and calculated motor current Im is supplied to the motor 20.
The control unit 30 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by a program within the CPU are shown in FIG. 2.
The functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque T detected by the torque sensor 10 is phase-compensated by a phase compensating section 31 and then inputted to a steering assist command value calculating section 32, and the velocity V detected by the velocity sensor 12 is also inputted to the steering assist command value calculating section 32. Based on the inputted steering torque T and velocity V, the steering assist command value calculating section 32 decides a steering assist command value I that is a control desired value of the current supplied to the motor 20 with reference to an assist map. The steering assist command value I is inputted to a subtracting section 30A and a differential compensating section 34 that is a feed-forward system for enhancing the response speed. A deviation (I-i) of the subtracting section 30A is inputted to a proportional calculating section 35 and an integral calculating section 36 that is used to improve characteristics of the feedback system. The output of the proportional calculating section 35 is inputted to an adding section 30B. The outputs of the differential compensating section 34 and the integral calculating section 36 are also inputted to the adding section 30B. A current control value E that is an addition result obtained by the adding section 30B, is inputted to a motor driving circuit 37 as a motor driving signal. Electric power is supplied to the motor driving circuit 37 from the battery 13 through the ignition key 11 and a fuse 33. A motor current value i of the motor 20 is detected by a motor current detecting circuit 38, and then the motor current value i is inputted to the subtracting section 30A to be fed back.
A general configuration example of the motor driving circuit 37 will be described with reference to FIG. 3. The motor driving circuit 37 comprises a PWM control section 37A and an inverter 37B. The PWM control section 37A comprises a duty calculating section 371 that calculates duty command values D1˜D6 of PWM signals of three phases according to a given expression based on the current control value E and a gate driving section 372 that drives gates of FETs as switching elements based on the duty command values D1˜D6 of the PWM signals and simultaneously switches on/off after performing a dead time compensation. The inverter 37B comprises a three-phase bridge of FETs and drives the motor 20 by being switched on/off based on the duty command values D1˜D6 of the PWM signals.
In such a motor driving apparatus, in the case of driving a multi-phase brushless motor by an inverter, it is necessary to figure out the rotor position (i.e. phases of the motor) and then sequentially switch the energized status to each phase according to phases of the motor. Motor driving apparatuses that the rotor position (i.e. the phases of the motor) is/are generally detected by a rotor position sensor such as hall elements or a resolver and switching of the inverter (FETs) is performed according to the detected rotor position, are put into practical use. Further, a motor driving apparatus that relational expressions between current value flowing in each coil of the motor and phases of the motor are known, the current value of each coil phase is measured and the phases of the motor are detected based on these current values, is proposed.
For the configuration of the current feedback system and the motor position calculation as described above, it is necessary to measure the current value of each coil phase. On the other hand, size reduction and weight saving of an electric power steering apparatus, and unification of current detectors (i.e. current sensors) as one item of cost reduction, are requested.
A configuration example of an inverter connected to a single current detector will be described with reference to FIG. 4. In this configuration example, a motor 50 having three phases (A-phase, B-phase and C-phase), is PWM-controlled by 6 FETs of a bridge configuration, electric power is supplied to high-side FETs from a power supply (a battery) 51, and a single current detector 60 is connected to power supply output side (earth side) of the bridge configuration. In this case, the current flowing in the current detector 60, that is, the detected motor current, varies depending on ON/OFF state of each FET. As an example, FIG. 5 shows a current path in the case that high-side FET of A-phase is in ON state (low-side FET of A-phase is in OFF state), and high-side FETs of B-phase and C-phase are in OFF state (low-side FETs of B-phase and C-phase are in ON state). Further, FIG. 6 shows a current path in the case that high-side FETs of A-phase and B-phase are in ON state (low-side FETs of A-phase and B-phase are in OFF state), and high-side FET of C-phase is in OFF state (low-side FET of C-phase is in ON state). From FIG. 5 and FIG. 6, it is clear that a sum of phase currents of phases that their high-side FETs are in ON state, appears in the current detector 60 as a current. In the case that the current detector 60 is connected to power supply input side of the inverter, this is similar.
Based on the above-described thing, it is clear that the detection of phase currents of three phases becomes possible by detecting the current by using the current detector 60 in the case that one phase is in ON state and two phases are in ON state and then utilizing a characteristic that a sum of phase currents of three phases is zero. In the case of FIG. 5, current IA of A-phase is detected by the current detector 60. Further, in the case of FIG. 6, a sum of current IA of A-phase and current IB of B-phase is detected by the current detector 60. Since there is a relationship that IA+IB+IC=0, current IC of C-phase can be detected based on IC=−(IA+IB). That is to say, even depending on the current detection by using the single current detector 60, by utilizing the characteristic that a sum of phase currents of three phases is zero, it is possible to detect phase current of each phase.